The present invention is directed generally to semiconductor devices. More specifically, the invention is directed to gate dielectrics. Yet more specifically, the invention is directed to the use of silicon nitride (SiN.sub.3) as a gate dielectric.
The use of SiN.sub.3 as a gate dielectric in a metal-oxidesilicon (MOS) device is highly desireable because of certain characteristics of the SiN.sub.3 layer, namely its higher dielectric constant (as compared to that of silicon dioxide), its imperviousness to water vapor and ion diffusion, and radiation shielding. As a consequence, the use of SiN.sub.3 as a gate dielectric in a MOS device has been pursued over the past 25 years or so.
However, in thin film form films of SiN.sub.3 having a thickness of less than about 1000 angstroms, SiN.sub.3 is generally unstable due to charge injection from the silicon into the nitride upon application of a voltage across an electrode formed on the SiN.sub.3 film and a silicon substrate upon which the film was deposited. The charged is trapped in the nitride and this results in a shifting of the threshold voltage of the metal-nitride-silicon (MNS) structure. By reversing the polarity of the applied voltage the charge can be transferred from the nitride to the silicon with a resulting converse shift of the threshold voltage in the opposite direction. The MNS structure, because of this phenomenon, is a variable threshold field effect device. The device is also known as a metal-nitride-oxide-semiconductor (MNOS) nonvolatile memory device. The phenomenon of the reversing thresholds thus is a hysteresis effect.
The hysteresis effect was discovered circa 1967 by H.A.R. Wagener of Sperry Corporation. This discovery led to the development of MNOS devices and the nonvolatile semi-conductor memory technology supported heavily by the United States military.
Until now, the only way known to eliminate the hysteresis in MNOS structures was to grow a sufficiently thicker, i.e., greater then about 1000 angstroms, layer of oxide between the nitride and the silicon substrate. But, such a structure has the disadvantage of an oxide layer which is sensitive to radiation and effectively neutralizes the advantages of the high dielectric constant of the silicon nitride.
The earliest MNOS memory devices were fabricated using silicon nitride deposited by means of atmospheric pressure chemical vapor deposition (APCVD). Accordingly, most published data on MNOS devices concerns APCVD silicon nitride. However, most silicon nitride depositions are currently performed by means of low pressure chemical vapor deposition (LPCVD) systems because LPCVD films are more uniform in thickness, are more nearly stoichiometric, contain less hydrogen, and contamination by oxygen is less common.
Since both of these deposition methods are performed at high temperatures, i.e., at temperatures greater than about 700.degree.C., there are difficulties in processing of radiation-tolerant peripheral MOS circuits. But, recent studies have suggested that by depositing silicon nitride by means of plasma-enhanced chemical vapor deposition (PECVD), a low temperature deposition, i.e., temperatures below about 400.degree.C., can be successfully used in a MOS structure. Studies have shown a strong dependence of some electrical properties of these PECVD films on deposition parameters, and have concluded that properties of PECVD silicon nitride which are important for nonvolatile memory device operations, are comparable to those of high temperature CVD silicon nitride. This result is not surprising in view of published reports of results for PECVD silicon dioxide and suggestions that low temperature epitaxy may be realizable.
Charge trapping and transfer properties of silicon nitride used in MNOS non-volatile memory devices are thought to be associated with excess silicon and hydrogen in the silicon nitride film. It has been shown that charge transfer in LPCVD silicon nitride films increases with excess silicon. A further increase has been produced by removing hydrogen by heating the device above the deposition temperature. Hydrogen implantations followed by annealing at about 500.degree.C. or rehydrogenation of annealed films in hydrogen plasma decreases the charge transfer and enhances charge trapping and retention due to the formation of Si-H bonds. In one study, it was demonstrated that Si--H bond density, which can be varied by controlling deposition parameters, has a significant impact on the physical and memory performance of "as deposited" PECVD silicon nitride.
Conventional post-deposition furnace annealing above about 500.degree.C. of the films results in a severe loss of hydrogen and affects the physical and memory properties of the films. However, reported results for PECVD nitride suggest that for annealing temperatures of less than about 500.degree.C., hydrogen can transfer from nitrogen to silicon and annealing of disorder in the films is possible. It has also been demonstrated that memory properties of MNOS capacitors can be enhanced by annealing at temperatures up to 500.degree.C. with the most significant improvement occurring for about 30 minutes at 475.degree.C.
Recently, rapid thermal processing (RTP) has received much attention as a tool for activating implants, growing thin insulated films, nitridation, and annealing of encapsulant films and gate dielectrics. RTP offers advantages over conventional furnace annealing, the most notable advantage being precision in controlling of annealing time and temperature for short thermal cycles. Consequently, RTP permits slow out-diffusion and/or redistribution of hydrogen during the annealing of silicon nitride. As noted previously, hydrogen plays an extremely important role in silicon nitride used as the trapping layer in an MNOS memory device.